Method of producing ultrasonic energy absorbing material



United States Patent U.S. Cl. 264111 2 Claims ABSTRACT OF THE DISCLOSURE A method of making an acoustic absorptive load for a piezoelectric vibrator in which a mixture of metal particles and a liquid epoxy binder are pressed in a mold to expel excess binder, and then curing the binder. The desired impedance of the load is determined by the pressure applied during or prior to the curing of the binder.

BACKGROUND OF THE INVENTION Ultrasonic transducers are commonly used to convert electrical signals into bursts of ultrasonic vibrations for use in industrial and medical applications. It is generally essential that the ultrasonic vibrations emanate from only one surface of the transducer and, therefore, that ultrasonic vibrations emanating from other surfaces of the transducer be absorbed. Undesirable vibrations from an ultrasonic transducer are conventionally absorbed by a matrix structure which includes metal particles in a binder and which is acoustically coupled to receive and absorb the undesirable ultrasonic vibrations. However, mismatches in the coupling of ultrasonic vibrations to the absorptive load produce undesirable reflections at the interfaces of acoustic media of dilferent acoustic impedances.

SUMMARY OF THE INVENTION Accordingly, mismatches in the coupling to an absorptive load are substantially eliminated by forming the load in accordance with the process of the present invention to have an acoustic impedance which substantially equals the acoustic impedance of the ultrasonic transducer. The acoustic impedance of the load is altered by altering the density of the matrix structure which forms the load.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the method of forming an absorptive load for an ultrasonic transducer in accordance with the present invention, a mixture of particles of a metal such as molybdenum, iron or tungsten, or the like (acoustic impedances typically greater than 45 in a system of units for which the acoustic impedance of water=l.5), and a suitable liquid binder such as epoxy resin is prepared in a ratio of materials which contains about twice the epoxy resin desired to have present in the finished material. The final matching of the acoustic impedances is accomplished according to the present invention by exerting pressure on the uncured volume of matrix materials causing about half of the uncured binder to be expelled from the mold used to hold the matrix materials during the pressing process. Thus, to match the acoustic impedance of an utrasonic vibrator element formed of a piezoelectric material such as barium titanate (acoustic impedance=24 in a system of units for which the acoustic impedance of water=1.5), or lead rnetaniobate (acoustic impedance=17,) the matrix material may be prepared using tungsten particles (specific gravity=l9) of about 4 to 6 micron size and liquid, uncured epoxy in a ratio by weight of less than about 12 parts tungsten to one part epoxy resin (for example, Eccogel 1265, distributed by Emerson & Cumin Company).

In the above formulation of the matrix material, the acoustic impedance of the cured, unpressed material is much lower than the acoustic impedance of the piezoelectric material it is intended to match. To obtain an acoustic impedance which matches the piezoelectric material, the liquid matrix material is loaded into a mold of suitable shape and is pressed to expel about half of the initial amount of the epoxy binder and to orient the metal particles in close contact throughout the volume of the mold. The mold may be of conventional design with suflicient differences in the dimensions of mating surfaces to allow excess epoxy to escape from the mold. This step of pressing out the excess epoxy increases the density and the stiffness, and, hence, the acoustic impedance of the matrix material in the mold. The pressure on the matrix material in the mold may be maintained during the curing cycle of the epoxy or it may simply be maintained only long enough to expel the excess epoxy prior to its curing cycle. The pressed and cured matrix material has been found to have an acoustic impedance which is typically twice as great as the acoustic impedance of similar types of acoustic load materials which are made without pressing or other means of compaction before or during the epoxy curing cycle.

The acoustic impedance of the absorptive load may be varied in accordance with the present invention over a range of values from about 10 to 25 simply by altering the pressure applied to the liquid matrix material in a mold to expel a certain amount of excess binder material. A few tests at various molding pressures exerted on selected formulations of matrix materials may thus pro vide sufiiicient data for selecting the matrix material formulation and molding pressure required to produce an absorptive load having an acoustic impedance which matches, typically within five percent, the acoustic impedanee of a given piezoelectric material.

I claim:

1. A method of producing an acoustic absorptive load for a piezoelectric vibrator, said load having a desired acoustic impedance and consisting of metal particles and a liquid resin binder curable to a solid. state, comprising:

(a) preparing a matrix of said metal particles with an amount of said liquid resin binder in excess of the resin binder in the prepared load,

(b) placing said matrix within a mold,

(c) pressing said matrix to expel a portion of said liquid resin from said matrix while increasing the density and stifiness of said matrix, and

(d) curing said liquid resin binder remaining in said matrix to a solid state to produce said absorptive load; said acoustic impedance in said produce load being determined by the pressing as applied in above step (c).

2. The method as in claim 1 wherein the metal particles and liquid binder combined in the'first-named step are combined in a ratio by weight of metal particles to binder less than about 12:1 for particles of metal having a specific gravity greater than 18.

References Cited UNITED STATES PATENTS 2,707,755 5/1955 Hardie et al. 310-82 2,881,336 4/1959 Elion 3108.2 3,403,271 9/1968 Lobdell et al. 3l08.2

ROBERT F. WHITE, Primary Examiner J. R. HALL, Assistant Examiner US! C X-R' 310-82 

